The differential action fire detector or the rate-of-rise action fire detector as commonly called in the trade having a heat-sensitive part which comprises an air chamber provided with a vent and a diaphragm is well known. The fire detector of this type works when the ambient temperature rises at a rate greater than a predetermined rate of temperature rise.
The combination type fire detector, which is a rate-of-rise action fire detector having further incorporated therein a bimetal heat sensor that functions at a predetermined temperature, is disclosed, for instance, in Japanese Laid-Open Utility Model Publication No. 12309/80. The fire detector of this type is advantageous in that it is able to give an alarm both when the temperature rapidly rises and when the temperature rises to a predetermined temperature although not so rapidly.
However, the bimetal heat sensor is rather expensive, as a mechanism for sensing a predetermined temperature. Moreover, its performance is not so precise and reliable over many years. Also, it is rather bulky and it is impossible to convert a conventional rate-of-rise action fire detector into a combination type detector by simply incorporating a bimetal heat sensor therein. A combination type fire detector must be redesigned for the incorporation of a bimetal heat sensor. That is, in order for a bimetal heat sensor to function efficiently, it must be fixed to the heat-sensing plate of the air chamber of the rate-of-rise action fire detector. Therefore, a new heat-sensing plate therefor must be designed.
Rather recently, a new heat-sensitive semiconductor element called "heat-sensitive thyristor" has been developed, which is available, for instance, from Mitsubishi Electric Corporation under the tradename "Thermosensor". This semiconductor is now being used as a heat sensor element. The "Thermistor", a well-known conventional heat-sensitive semiconductor element changes its resistance with change in temperature. In other words, it is a heat-sensitive resistor element, and does not perform sharp switching action. The above-mentioned heat sensor element, the "heat-sensitive thyristor", is a semiconductor element, which performs switching action at a predetermined temperature like a bimetal heat sensor, but much more precisely. That is, the switching temperature can be altered by changing the resistance value between the gate and the anode, and therefore, it is more suitable than the conventional thermistor, for use in a fire detector.